Bacteriophage therapy. Bacteriophage (phage) have been used for clinical applications since their initial discovery at the beginning of the twentieth century. However, they have never been subjected to the scrutiny--in terms of the determination of efficacy and pharmacokinetics of therapeutic agents--that is required in countries that enforce certification for marketed pharmaceuticals. There are a number of historical reasons for this deficiency, including the overshadowing discovery of the antibiotics. Nevertheless, present efforts to develop phage into reliable antibacterial agents have been substantially enhanced by knowledge gained concerning the genetics and physiology of phage in molecular detail during the past 50 years. Such efforts will be of importance given the emergence of antibiotic-resistant bacteria. Single amino acid substitution in a lambda phage major capsid protein is sufficient to confer a capacity to remain in a mammalian circulatory system. Proposals to explain clinical failures of phage therapy include the interactions of phage with the immune system1. However, in experiments with germ free mice, with no detectable adaptive immune system antibodies to lambda phage, phage titers in the circulatory system of mice were found to decrease exponentially by more than 109pfu within 48 hours of intraperitoneal (i.p.), intravenous or oral phage administration2. Based on these observations, lambda phage mutants were selected, using a serial passage technique, with a 13,000 to 16,000 fold greater capacity to remain in the Balb/C mouse circulatory system 24 hours after i.p. injection3. These "long-circulating lambda phage, called lambda phage, had at least three mutations including one in the major phage capsid (E) protein the latter resulting in the change of a glutamic acid to a lysine at residue 158. In the current experiments we demonstrate that this specific substitution in E protein is sufficient to confer the full "long-circulating" phenotype to wild type lambda phage. This recombinant lambda Argo phage, created by maker rescue, with the single E158K substitution and its isogenic parental wild type strain will provide guidance for the development of more effective antibacterial therapeutic phage strains and they may be useful in studies of the innate immune system.